This invention relates generally to radio frequency power amplifiers, and more specifically to variable power radio frequency power amplifiers, and is particularly directed towards providing a high efficiency variable power radio frequency amplifier.
The efficiency of any amplifier is determined by its ability to appropriately amplify an input signal without consuming an excessive amount of energy. Typically, the energy consumed by an amplifier is chiefly attributed to the bias currents drawn by the amplifier. Ordinarily, higher bias current consumption is directly related to the heat energy that is generated during the amplifier operation. Accordingly, a more efficient amplifier will generate less heat and consume less current when amplifying an input signal.
Class C is the most efficient operation for RF amplifiers. To ensure biasing beyond cutoff for class C operation, the transistor base bias voltage is maintained below threshold (less than 0.7 Vdc) for a bipolar silicon transistor. The lower the base voltage, the more efficient the amplifier (0 Vdc or lower is the best).
A significant detriment of amplifiers having variable output power is their inability to maintain efficient operation across the full range of their variable output power. Conventional variable power amplifiers are customarily designed to maximize efficiency at the maximum power level to be used. Reduced efficiency at other power levels is tolerated based upon the expectation that the amplifier will ordinarily operate at the more efficient power level.
For variable power amplifiers operating in a battery powered transmitter, reduced amplifier efficiency decreases the battery life. This is particularly detrimental in applications where the amplifier does not operate predominantly at a single power output level.
Accordingly, a need exist in the art for a variable power amplifier having an improved efficiency over its variable power range to maximize battery life of a battery powered transmitter.